MOLLUS CA.
THE history of the great division of animals to which the term MoLLuseA is now exclusively confined, was investigated in a very imperfect manner by the earlier naturalists. They attended merely to the characters furnished by the external appearance, and consequently formed their systematical divisions without regard to the natural affinities of the animals—affinities which can only be traced by an examination of the structure and functions of all the organs.
The first general attempt at classification, worthy of notice, appears in the twelfth edition of the Systema Nature of Linrmus. In this work, all the animals which were considered by the ancients as Exanguine ous, and termed by the more recent naturalists, Inver tebral, were, with the exception of insects, included in his sixth and last class, which he denominated Vermes, and assigned to it the following distinguishing charac ters.—Cor, uniloculare, inauritum ; Sanie frigida. Spi racula obscura. Maxilla multifarix, varix variis. Penes varii hermaphroditis Androgynis. Sensus : Tentacula (caput nullum, vix oculi, non cures, nares.) Tegmenta calcarea aut nulla, nisi spinet. Fulcra : nulli Pedes aut Pinnx. This class of vermes was again divided into four orders—Intestina, animalia simplicia, absque artu bus, nuda. Mollusca, animalia simplicia, nuda (absque testa inhabitata,) artuhus instructa. Testacea, animalia Mollusca, simplicia, domo, sxpius calcarea, propria obtecta. Lithophyta, animalia Mollusca, composita.
Corallium calcareum, fixum, quod inxdificarunt animalia affixa. Zoophyta animalia composita, effiorescentia. Stirps vegetans, metamorphosi transiens inflorens ani mal. The second and third orders, Mollusca and Tes tacea, include the animals to which our attention is to be directed in this article.
This systematical arrangement of "Animus, while it contributed greatly to enlarge the number of species, had a tendency to divert the attention from the examina tion of their structure. The external form was exclu sively employed to furnish the distinguishing characters, and was therefore chiefly regarded by the student, in reference both to genera and species. The relation be
tween the external appearance, and the internal structure, not having been previously determined, these characters were obviously artificial, and the unnatural combinations which resulted from their employment, displayed them selves in every genus of any extent. In the same genus animals were to be found which respire in air, associated with those which perform the same function by means of gills. in water ; or animals whose gills are like leaves placed externally, with those having their gills in an in ternal cavity. Such incongruous combinations chiefly prevail among the vermes testacea, where the shell is exclusively employed in the determination of genera and species. There is another imperfection in this system, arising from the separation of the naked and testaceous mollusca into distinct orders. There is no such line distinction observed by nature ; and very considerable difficulties occur in its practical application as an arti ficial arrangement. Not a few animals included in the testacea, as the Bulla aperta, have the shell so concealed under the skin, that it can only be rendered visible by the separation of the softer parts ; and several animals, included among the naked mollusca, arc covered, in par ticular places, by a corneous or shelly plate, as the Aplysiae. In consequence of these circumstances, genera which arc nearly related are placed in different orders, while those which possess few common properties are grouped together.
Independent of these objections, which have been made against the Linnxan classification of molluscous animals, it is still pertinaciously adhered to by many British naturalists. This influence of the idolum theatri is the more to be regretted, as it has retarded the pi o gress of science, limited the field of observation, and prevented us from availing ourselves of the improve ments which have resulted from the labours of our con tinental neighbours.